Legend:

map_meta_cap is not prepared to handle missing or invalid post ID's. To duplicate this, delete a post from the posts table without deleting its attached comments, then visit the admin dashboard with the recent comments widget visible.

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map_meta_cap is not prepared to handle missing or invalid post ID's. To duplicate this, delete a post from the posts table without deleting its attached comments, then visit the admin dashboard with the recent comments widget visible.

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Update: this isn't something that happens naturally in WordPress core, but is something plugins are able to do directly to the database, which results in orphaned comments. The attached patches only suppress debug notices PHP spits out, and does not directly address the issue of what to do with orphaned comments, should a plugin choose to do this.